1. Field of the Invention
The present invention relates to a cable, particularly for carrying high voltage direct current, and suitable for either terrestrial or, in particular, submarine installations, having as insulating layer a material impregnated with an insulating fluid.
For the purposes of the present description and the claims, with the term xe2x80x9chigh voltagexe2x80x9d it is meant a voltage of at least 100 kV, preferably of at least 200 kV.
2. Related Art
For carrying high voltage direct current, either along terrestrial lines or, in particular, along submarine lines, use is made of cables, commonly known in the art as mass-impregnated cables, in which the conductor, covered with a first semiconducting layer, is electrically insulated by taping with an insulating material, generally paper or multi-layered paper/polypropylene/paper laminates, which is then thoroughly impregnated with a mixture having high electrical resistivity and high viscosity, generally a hydrocarbon oil treated with a viscosity-increasing agent. The cable then comprises a further semiconducting layer and a metal sheath, generally made from lead, which in turn is surrounded by at least one metal armouring structure and one or more protective sheaths made from plastic material.
Although characterized by high reliability in operation even at very high voltages (above 150 kV), mass-impregnated cables have some disadvantages, principally relating to the migration of the insulating fluid within the cable. In particular, during use the cable is subject, owing to variations in the carried current intensity, to thermal cycles which cause migration of the impregnating fluid in radial direction. This is because, when the carried current increases and the cable heats up, the insulating fluid decreases in its viscosity and is subject to a thermal expansion greater than that of all the other components of the cable. This causes a migration of the fluid from the insulating layer towards the exterior, and consequently an increase of the pressure exerted on the metal sheath, which is deformed in the radial direction. When the carried current decreases and the cable cools, the impregnating fluid contracts, while the metal sheath, being constituted by a plastic material (usually lead), remains permanently deformed. Thus there is a decrease of the pressure inside the cable, which causes formation of micro-cavities in the insulating layer, with consequent risk of electrical discharges and therefore of insulation piercing. The risk of piercing increases with an increase of the insulating layer thickness and therefore with an increase of the maximum voltage for which the cable has been designed.
For carrying direct current at high voltage, pressurized cables have been developed, wherein the insulating layer impregnated with the insulating fluid is subjected to a pressure greater than atmospheric pressure, generally above 14 bars, by introducing a pressurized gas, for example nitrogen. Another solution for carrying high voltage direct current consists of fluid oil cables, in which insulation is provided by a pressurized oil with low viscosity and high resistivity (under hydrostatic head). These solutions, although very effective in preventing formation of micro-cavities in the cable insulation, have various disadvantages mainly related to construction complexity, and in particular they cause a limitation of the maximum admissible cable length (generally of not more than 50-100 km). This limitation of the maximum length is a serious drawback, especially in the case of submarine installations, where the required lengths are usually very great.
In Patent GB-2,196,781 a direct current mass-impregnated cable is described, in which a reinforcing structure is present consisting of an elastic tape wound tightly around the metal sheath so as to compensate for radial expansions and contractions of the impregnating oil and consequently to prevent formation of micro-cavities in the insulation. The tape consists of an elastic material of a metal or polymer type, characterized by mechanical hysteresis cycles of the closed type, with a low permanent deformation after thermal cycles.
Patent EP-233,381 describes a high voltage mass-impregnated cable comprising a pressure body consisting of a supporting layer, disposed on the outside of the metal sheath, around which is wound, in at least two overlapping layers, a thin elastic tape made from metal or polymer material. The presence of the supporting layer interposed between the metal sheath and the elastic tape acts as a xe2x80x9cbeddingxe2x80x9d, preventing the tape, which is wound with high tension to exert a sufficient containing action on the metal sheath, from damaging the sheath or from eventually causing it to fracture. The risk of fracture increases when the cable overheats, partly owing to the fact that the lead constituting the sheath becomes more malleable, and partly because the cable expands and therefore the tape tension increases.
In the Applicant""s perception, both the solutions proposed in the aforesaid patents GB-2,196,781 and EP-233,381 have several drawbacks arising from the fact that the containing tape is disposed on the outside of the metal sheath. Indeed, as explained above, the tension exerted by the tape may damage the sheath and even cause it to fracture, and on the other hand the interposition of a supporting layer between the tape and the sheath, as proposed in patent EP-233,381, not only makes the cable construction more complex, but also decreases effectiveness of the containing action exerted by the said tape on the sheath.
Moreover, the high pressures required to achieve the desired return of the metal sheath during the thermal contraction phase cause a rapid wear of the containing tape, owing to the friction between tape and sheath and between the turns of the tape itself. Finally, the containing tape, being applied externally to the sheath, is subject to the action of external agents, particularly water which may infiltrate under the armour, and this causes a degradation over time of its elastic and mechanical properties.
The Applicant has now found that it is possible to achieve an effective action of containing of the insulating fluid, thus preventing formation of micro-cavities inside the impregnated insulating layer, by winding around the insulating layer beneath the metal sheath at least one tape made from elastomeric semiconducting material, which exerts a compressive action on the insulating layer and acts as a barrier with respect to the radial migration of the insulating fluid without damaging the metal sheath. By contrast with the solutions known in the art, according to the present invention the action of containing of the impregnating fluid is exerted directly on the insulating layer, so that the formation of micro-cavities in the insulation is avoided, even if empty spaces are formed between the cable core and the metal sheath.
The semiconducting properties of the elastomeric material ensure electrical continuity by preventing the creation of potential differences between the core of the cable and the metal sheath, which would lead to electrical discharges and consequently to perforation of the cable. Moreover, the material constituting the tape has a high physical-chemical resistance to the degrading action exerted by the components of the insulating fluid, both in the cold state and, in particular, at the cable operating temperature. This is because the insulating fluid, usually consisting of products of the hydrocarbon type, may cause, especially when hot, a swelling of the elastomeric material and consequently a gradual degradation of elastic and semiconducting properties of the material, with a loss of functionality of the containing layer over time.